Valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having a gate opening extending transversely therethrough is disposed in the gate cavity. A valve stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate cavity of the valve body is covered by a bonnet having an axial bore through which passes the valve stem.
Such a gate valve is associated to a valve operator assembly for selectively driving the valve stem up and down in order to close and open the gate valve. A gate valve may be manually actuated. In this case, the valve operator assembly generally comprises a transmission mechanism to convert the rotational motion of a hand-wheel into axial motion of the valve stem. To quickly open and close the gate valve with a minimum number of turns, the transmission mechanism may be a ball screw mechanism or a planetary roller screw mechanism in order to reduce the operating torque, for instance manual hand-wheel torque or powered with electric drive for surface valves or with remote operating vehicle (ROV) or electric actuation for subsea valves. For more details, it is possible for example to refer to the patent EP-B1-1 419 334 (SKF).
A balance system is generally provided on the valve body of the gate valve to prevent a back-driving of the transmission mechanism with the upward force exerted by the fluid. Under fluid pressure, the gate valve can effectively be inadvertently opened or closed. The balance system may comprise a balance stem disposed on the valve body and which is exposed to fluid pressure to offset or balance the force exerted on the gate.
When operating a gate valve without using such a balance stem, the force of the fluid helps in one direction a movement of the valve stem and the screw of the transmission mechanism whereas the force is against a movement in the opposite direction. For instance, when the valve is vertical and the drive input is uppermost, the upward force of the fluid helps an upwards movement of the valve stem and the screw. Accordingly, the required torque on the rotational drive input, such as manual hand-wheel, is usually much higher for the movements in one direction compared to the movements in the opposite direction.